Why a 24-knot Dreadnought does not erase battlecruisers

Would a 24-knot HMS Dreadnought make the battlecruiser concept unnecessary?

No. Battlecruisers would move toward 28 knots, and battleships could not routinely match that speed until small-tube boilers matured.

A 24-knot HMS Dreadnought would not immediately merge the battleship and battlecruiser types because the latter still required a clear speed advantage. The next battlecruisers would likely aim for roughly 28 knots. Queen Elizabeth later demonstrated that battleships could reach about 24 knots, but routine higher speeds awaited small-tube boilers. The technological timing therefore delays a full convergence of the types.